MnCo2O4h-BN
6.3.7 Electrochemical Measurements
To determine the OER performance of electrocatalysts, linear-sweep voltammetry measurements (LSV) were performed. Figure 6.9 (a) shows the LSV curves of respective electrocatalysts, measured at the scan rate of 1 mV/sec, and the overpotential of each electrode was determined at the current density of 10 mA/cm2 (ƞ10). The ƞ10 value of pristine C-MCO was found to be 390 mV/cm2, while for h-BN nanosheets ƞ10 was calculated to be 425 mV/cm2, and h-BN nanosheets modified C-MCO/h-BN electrode shows lower ƞ10 value of 240 mV/cm2. The OER performance of the C-MCO/h-BN electrode was determined with 1.20 mg/cm2 and 0.11 mg/cm2 loading of C-MCO and h-BN respectively to understand the mass activity. The OER performance of C-MCO/h-BN was compared with the benchmark catalyst (RuO2),with the same loading amount of catalyst and under similar experimental conditions. RuO2 displays ƞ10 = 290 mV, indicating modified electrode displays better OER performance than the traditional metal oxide. Figure 6.9 (b) shows the optimised OER activity of C-MCO/h-BN with different loading amounts of h-BN nanosheets over C-MCO. The third cycle of h-BN nanosheets loading over C-MCO shows maximum OER activity. To examine the
-33.1 h-BN
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Chapter 6 MnCo2O4/h-BN reproducibility of the modified electrode, three consecutive linear sweep voltammetry measurement of the same MnCo2O4/h-BN electrode was carried out. The three consecutive measurements of MnCo2O4/h-BN, shown in Figure 6.9 (c) confirm the reproducibility of OER activity by the modified electrode.
Figure 6.9. (a) Linear sweep voltammetry (LSV) curves of C-MCO/h-BN, RuO2, C-MCO and h-BN nanosheets at the scan rate of 1 mV/sec in purified 1 M KOH. (b) Optimised LSV curves of C-MCO/h-BN electrode with different loading amount of h-BN nanosheets over C-MCO. (c) Reproducibility test of C-MCO/h-BN.
Further to determine the OER kinetics of the electrocatalysts, Tafel slope values were calculated. The Tafel slope is shown in Figure 6.10 (a) of each catalyst was calculated from their respective LSV curves. The metal-free surface-modified C-MCO/h-BN shows a lower Tafel slope value of 66 mV/dec, while pristine C-MCO displays a Tafel slope value of 207 mV/dec. Tafel slope values for h-BN nanosheets and RuO2 were calculated to be 106 mV/dec and 78 mV/dec respectively. The Lower Tafel slope value of the composite indicates, C- MCO/h-BN displays faster OER kinetics than the precious metal oxide (RuO2) and pristine C- MCO under harsh alkaline conditions.13 To get insight into enhanced OER performance of the modified electrocatalyst, electrochemical impedance spectroscopy (EIS) measurement was done at 1.6 V vs RHE. Charge transfer resistance (Rct) of the electrocatalysts was determined from the Nyquist plot, as shown in Figure 6.10 (b). Rct value of the composite C-MCO/h-BN was calculated to be 3.9 Ω, while for pristine C-MCO and h-BN, Rct was found to be 23.9 Ω and 5.7 Ω respectively. The lower Rct value of the modified electrode indicates the smooth transfer of charge carriers at the interface of composite (C-MCO/ h-BN) and the electrolyte.33
1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 0
10 20 30 40 50 60 70 80 90 100
MnCo2O4/h-BN-1st cycle
310 m v
Current density mA/cm2
Potential (V vs RHE) 240 m
v 290 m
v
330 m 1 mv/sec v
MnCo2O4/h-BN-4th cycle MnCo2O4/h-BN-2nd cycle MnCo2O4/h-BN-3rd cycle
1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 0
10 20 30 40 50 60 70 80 90 100
29 0 mv MnCo2O4/h-BN RuO2
MnCo2O4 h-BN
1 mv/sec 240 m
v
390 mv 425 mv Current density (mA/cm2)
Potential (V vs RHE) 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0
0 10 20 30 40 50 60 70 80 90 100
MnCo2O4/h-BN 1stRun 2ndRun 3rdRun
Current density (mA/cm2)
Potential (V vs RHE) 1 mv/sec
(b) (c)
(a)
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123 Part of this chapter has been published in ACS Appl. Energy Mater: 2022, 5, 1551
Figure 6.10. (a) Tafel plots of C-MCO/h-BN, RuO2, C-MCO, and h-BN nanosheets. (b) Nyquist plot of C- MCO/h-BN, C-MCO, and h-BN at 1.6 V vs RHE.
Figure 6.11 shows cyclic voltammetry of pristine C-MCOelectrode, displayingredox process taking place in it on the application of potential. In the voltammogram of C-MCO, on the applying potential across the electrode, Mn3+get oxidised to Mn4+, and Co2+ is oxidised to Co3+ and vice-versa.
Figure 6.11. Cyclic voltammogram showing (Co2+/ Mn3+ ↔ Co3+/Mn4+) redox cycle in bare C-MCO.
To further understand the cause for the enhancement in OER activity ofC-MCO/h-BN with the incorporation of h-BN nanosheets, electrochemical surface area (ECSA) calculations were done. ECSA of an electrocatalyst is indicative of the number of electrochemically active sites present. The ECSA value of C-MCO/h-BN, C-MCO, h-BN, and RuO2 were calculated
0.0 0.2 0.4 0.6 0.8 1.0 1.2
0 100 200 300 400 500
MnCo2O4/h-BN RuO2
Overpotential (mv)
log(j) (mA/cm2) MnCo2O4 h-BN
207 mV/dec 106 mV/dec
66 mV/dec 78 mV/dec
0 5 10 15 20 25 30 35 40
0 5 10 15 20 25 30 35
40 MnCo2O4/h-BN
h-BN MnCo2O4
-z" ()
z'()
Electrode Rct() MnCo2O4/h-BN 3.9
h-BN 5.7
MnCo2O4 23.9
-Z" (k)
Z' (k)
(b) (a)
0.9 1.0 1.1 1.2 1.3 1.4 1.5 -0.05
0.00 0.05 0.10 0.15
Current density mA/cm2
Potential (V vs RHE) MnCo2O4
Co2+ Co3+
Mn3+
Mn4+
Mn4+
Mn3+
Co3+
Co2+
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Chapter 6 MnCo2O4/h-BN from the cyclic voltammetric measurements as shown in Figure 6.12 (a, b, c, d) respectively.
Cyclic voltammetric measurements of each electrode were done at different scan rate (1-6) mV/sec. For maximum accuracy in Cdl estimation, we have avoided faradaic current zone and did cyclic voltammetry measurements of C-MCO and C-MCO/h-BN in the same potential range (0.965 – 1.025 V) vs RHE. They were measured at potentials sufficiently cathodic of the OER activity (non-faradaic region), and also away from metal cations redox features (Figure 6.11).34 The Cdl value of all the catalysts was estimated at 1.0 V vs RHE. ECSA is directly propositional to double layer capacitance (Cdl), consequently, ECSA of pristine C-MCO and C-MCO/h-BN can be directly estimated by calculating their Cdl value from their respective cyclic voltammogram. The difference in current density (janode - jcathode) of C-MCO/h-BN and C-MCO electrodes were plotted against the scan rate to calculate their respective slope values, whereas Cdl is half of the slope value, shown in Figure 6.12 (e).35
Figure 6.12. Cyclic voltammetry plot at different scan rates (1-6) mV/sec in the non-faradaic region of (a) C- MCO, (b) C-MCO/h-BN, (c) h-BN, (d) RuO2. (c) double layer capacitance (Cdl) plot of C-MCO/h-BN, pristine C-MCO, h-BN, (d) RuO2.
The Cdl value for surface modified heterostructure was calculated to be 30.6 mF/cm2, while pristine C-MCO shows Cdl value of 5.5 mF/cm2. The metal free h-BN nanosheets
0.96 0.97 0.98 0.99 1.00 1.01 1.02 1.03 -6
-4 -2 0 2 4 6 h-BN
Current density (A/cm2)
Potential (V vs RHE) 1 mV/sec 2 mV/sec 3 mV/sec
4 mV/sec 5 mV/sec 6 mV/sec 0.96 0.97 0.98 0.99 1.00 1.01 1.02 1.03
-0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4
Current density (mA/cm2)
MnCo2O4/h-BN
Potential (V vs RHE)
1 mV/sec 2 mV/sec 3 mV/sec 4 mV/sec 5 mV/sec 6 mV/sec 0.96 0.97 0.98 0.99 1.00 1.01 1.02 1.03
-0.15 -0.10 -0.05 0.00 0.05 0.10 0.15
Current density (mA/cm2)
Potential (V vs RHE) 1 mV/sec 2 mV/sec 3 mV/sec 4 mV/sec 5 mV/sec 6 mV/sec
MnCo2O4
(a) (b) (c)
0.96 0.97 0.98 0.99 1.00 1.01 1.02 1.03 -0.3
-0.2 -0.1 0.0 0.1 0.2 0.3
Current density (mA/cm2)
RuO2
1 mV/sec 2 mV/sec 3 mV/sec
4 mV/sec 5 mV/sec 6 mV/sec
Potential (V vs RHE) 1 2 3 4 5 6
0.0 0.1 0.2 0.3 0.4 0.5
J (mA/cm2)
Scan Rate (mV/sec) MnCo2O4/h-BN
MnCo2O4 h-BN
Cdl= 0.28 mF/cm2 Cdl= 5.5 mF/cm2
Cdl = 30.6 mF/cm2
1 2 3 4 5 6
0.0 0.1 0.2 0.3 0.4
RuO2
Cdl= 25 mF/cm2
j (mA/cm2)
Scan Rate (mV/sec)
(d) (e) (f)
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125 Part of this chapter has been published in ACS Appl. Energy Mater: 2022, 5, 1551
modified C-MCO heterostructure displays a ~5.5-fold increase in Cdl value. While Cdl value of pristine h-BN and RuO2 was calculated to be 0.28 mF/cm2 and 25 mF/cm2 respectively, shown in Figure 6.12 (e) and Figure 6.12 (f) respectively.
Turnover frequency (TOF) of C-MCO/h-BN, pristine C-MCO, and pristine h-BN electrodes were determined to estimate the number of oxygen molecules formed from the active sites of the electrocatalyst per unit time, at an operating potential of 1.53 V vs RHE, shown in Figure 6.13. To determine the TOF of the electrocatalyst for OER activity, the concentration of active sites (NS) must be quantified first. The anodic current values were plotted against different scan rates, from the cyclic voltammogram of C-MCO/h-BN and pristine C-MCO, where they show a linear relationship. The slope of the catalysts was calculated using equation 6.136, 35
𝑆𝑙𝑜𝑝𝑒( 𝐽𝑎𝑛𝑜𝑑𝑖𝑐
𝑆𝑐𝑎𝑛 𝑟𝑎𝑡𝑒) = 𝑛2𝐹2𝐴𝑁𝑆/4𝑅𝑇 --- (6.1)
n (the number of electrons transferred), T (absolute temperature), and R (ideal gas constant).
TOF was calculated using equation 6.237 𝑇𝑂𝐹 = 𝐽 ×𝐴
𝑛×𝐹×𝑁𝑆 --- (6.2) Herein, J (current density at particular overpotential (A/cm2)), A (surface area of the electrocatalyst (cm2)), n (number of electrons transferred to evolve a molecule of O2), F (Faraday constant (96458 C/mol)), and NS (concentration of active sites of the electrocatalysts (mol/cm2)).
Surface modified C-MCO/h-BN heterostructure displays TOF of 0.195/sec, while its pristine counterpart shows a lower TOF of 0.07/sec and pristine h-BN shows a TOF value of 0.042/sec. The ~2.8-fold enhancement in TOF in the composite is due to the high hole
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Chapter 6 MnCo2O4/h-BN extracting ability of negatively charged h-BN nanosheets, efficient OER kinetics, and enhancement in charge transfer kinetics at the working electrode-electrolyte interface.
Figure 6.13.Turnover frequency (TOF) of C-MCO/h-BN, pristine C-MCO, and pristine h-BN.